lesson.scss: define 'inline' class for images

Define `inline` class for images that should not be displayed as block
elements.

By appending `{:class="inline"}` or `{: .inline}` to the image definition
in Markdown, one can create an inline image that doesn't break the
current line and is embedded in the paragraph. Useful for showing
special symbols and hieroglyphs that we can't display by other means.

Author: maxim-belkin

assets/css/lesson.scss

@@ -258,6 +258,11 @@ article img {
     max-width: 100%;
 }
 
+article img.inline {
+    display: inline-block;
+    margin: auto;
+}
+
 article h2 {
   margin: 48px 0 16px;
   border-bottom: solid 1px #eaecef;